It is known that bevel gears can be machined using a grinding tool. So-called cup grinding wheels are frequently used in this case.
So-called discontinuous profile grinding is a grinding process according to the single indexing method. In particular, discontinuous profile grinding is used to manufacture plunged crown wheels. During the plunging of the cup grinding wheel, the profile of the cup grinding wheel is imaged in the material of the crown wheel to be manufactured. The corresponding method is also designated as the index forming method.
During the grinding of spiral-toothed bevel gears, the concave tooth flank of one tooth gap is produced using the outer peripheral surface and the convex tooth flank of the tooth gap is produced using the inner peripheral surface of the cup grinding wheel. If this is performed in two-flank cutting, also called completing, which is typical in the case of plunge grinding of crown wheels, both tooth flanks of the tooth gap are ground simultaneously. In one-flank grinding, in contrast, either only the concave tooth flanks or only the convex tooth flanks of the tooth gap are ground.
A large contact surface over the entire tooth width of the work piece results in this case in the plunge grinding of spiral bevel gears. Coolant liquid cannot reach the grinding zone here. Because of the large contact surface and the poor cooling, so-called grinding burn can occur on the tooth flanks of the work piece. In addition, problems can occur in the chip removal and the cup grinding wheel can clog with metal particles.
It is known that the plunging movement of the grinding wheel can have an eccentric auxiliary movement of the grinding wheel center point superimposed, in order to thus remedy the mentioned problems or to reduce their influence on the grinding process. Due to the mentioned superposition, the cup grinding wheel center point moves on an orbit around a center point. The radius of this orbit is designated as the eccentric stroke and is small in comparison to the radius of the cup grinding wheel. Because of this movement, the cup grinding wheel will only contact the work piece in one point, considered geometrically; in reality, however, it is a locally delimited region in which the contact occurs, because of the feed movement. The ratio of the eccentric speed to the speed of the cup grinding wheel is the so-called eccentric ratio.
The eccentric auxiliary movement can be produced in grinding machines by setting the eccentric ratio, or the eccentric speed, respectively, in the form of a fixed specification.
An undesirably large surface contact between the grinding disc and the crown wheel can be avoided by the eccentric movement. Details on the superposition of a cyclic eccentric movement can be inferred, for example, from German patent application publications DE 2721164 A and DE 2445483 A. The principle of the eccentric movement originates from the inventor Waguri, who made corresponding developments in the year 1967.
FIG. 1A shows a schematic illustration of the so-called Waguri approach, in which a cup grinding wheel 2 rotates around a wheel center point M1, which is offset relative to the center point M2 of a Waguri wheel 3 by a small distance e (referred to as the Waguri eccentricity). The eccentric ratio is defined as the speed of the eccentric divided by the speed of the cup grinding wheel 2. The cup grinding wheel 2 rotates at the angular velocity ω1 around the center point M1. The eccentric movement causes a circular movement around M2 for the center point M1 of the cup grinding wheel 2. This circular movement has movement components in the x direction and in the y direction.
In the case of otherwise constant ratios, the frequency of the contact of the cup grinding wheel 2 with the tooth flank, the location of this contact region on the cup grinding wheel 2, and the phasing of the contact of concave and convex tooth flanks in the case of two-flank cutting and the possible displacement thereof on the cup grinding wheel 2 are dependent on the selected eccentric ratio.
For example, if one presumes that in the case of an eccentric rotational angle of 0° (the 0° position is coincident with the y axis here), the contact between the cup grinding wheel 2 and the concave flank 5.1 of a tooth gap 5 occurs in the region 4 (see FIG. 1A), thus, after a rotation of the eccentric by 180°, the contact between the cup grinding wheel 2 and the convex flank (in FIG. 1A, the convex flank of the next tooth gap is identified by the reference sign 5.2) of the tooth gap 5 will occur.
If the eccentric ratio is 1, the eccentric rotates once during one revolution of the cup grinding wheel 2. The cup grinding wheel 2 contacts the concave flank 5.1 of the tooth gap 5 (at) 0° once and the convex flank of the tooth gap 5 (at 180°) once during each full revolution. The contact always occurs in the same region. If the eccentric ratio is 2, two contacts of the cup grinding wheel 2 with the concave flank 5.1 of the tooth 5 (at 0° and 180°) or with the convex flank of the tooth 5 (at 90° and 270°), respectively, thus occur during one full revolution of the cup grinding wheel 2. At an eccentric ratio of 0.5, the concave flank 5.1 is contacted at 0° and 720° and the convex flank is contacted at 360° and 1080°. These degree specifications respectively relate to a fixed coordinate system of the cup grinding wheel 2 and in the mentioned three special cases, there is no shift of the contact regions along the grinding wheel periphery from full revolution to full revolution of the cup grinding wheel 2.
In general, however, a shift of the contact region on the cup grinding wheel 2 occurs per full revolution, so that the entire grinding wheel periphery is used for the grinding machining of the work piece 1. The eccentric ratio which is predefined can also be a rational number Q. A practical example is an eccentric ratio of 0.7. In this case, the contact of the concave flank 5.1 would occur at 0° and 514.2857° (corresponding to 154.2857° on the grinding wheel periphery) and that of the convex flank at 257.1428° and 771.4286° (corresponding to 51.4286° on the grinding wheel periphery). If the cup grinding wheel 2 executes a plurality of full revolutions, the contact regions are displaced further and further and finally the entire grinding wheel periphery on the profile 8 is used for the grinding machining.
FIG. 1B shows a section along reference line X1-X1 through a part of the cup grinding wheel 2. The profile 8 of the cup grinding wheel 2 can be recognized in FIG. 1B.
Due to the superimposed eccentric movement, an (excessively) large surface contact between the outer periphery 8.1 of the profile 8 of the cup grinding wheel 2 and the entire surface of the concave flank 5.1 of the work piece 1 and the inner periphery 8.2 of the profile 8 of the cup grinding wheel 2 and the entire surface of the convex flank is avoided.
Details on a grinding method having superimposed eccentric movement are described, for example, in the document “Guidelines for Modern Bevel Gear Grinding”, H. J. Stadtfeld, revised May 2008, The Gleason Works, USA, on pages 14 and 15.
Grinding wheels are referred to in general hereafter, although in the concrete case these are mostly cup grinding wheels.
Studies have shown that grinding burn can nonetheless form on grinding wheels that are eccentrically mounted according to the Waguri approach. In addition, these eccentrically mounted grinding wheels can still also be clogged with metal residues. In addition, these grinding wheels can have an inadequate service life.